GEOLOGY OE THE MONT CENIS TUNNEL. 
339 
length of road over the pass corresponding to this direct dis- 
tance is nearly twenty-three miles. The valleys on each side 
are always fit for travelling, except immediately after serious 
floods, when the torrents coming down from the narrow gorges, 
chiefly on the French side, sometimes tear up and destroy the 
road. The Mont Cenis pass is, however, on the whole very 
approachable on both sides, and offers no particular or excep- 
tional difficulties. The railway now in operation and intended 
ultimately to pass through the tunnel and connect with the 
line already open from Susa to Turin, terminates at St. Michel, 
a small and miserable village, beyond which to Modana, the 
entrance of the tunnel, is a distance of upwards of ten miles. 
From St. Michel to Susa the mountain rail known as the “ Fell 
line ” completes the communication, and is an admirable sub- 
stitute for a railway of the ordinary kind ; but it must be 
entirely superseded when the tunnel is once opened. 
Geologically, the road beyond St. Michel towards Lansle- 
bourg, the last village before the ascent of the mountain, crosses 
an important and extensive portion of the metamorphosed ju- 
rassic rocks of Switzerland, including — (1) a series composed of 
quartzy conglomerate, quartzite, fine grained gritstone, and some 
argillaceous and calcareous schistose rocks with anthracite, cor- 
responding as a series, but not in detail, with the middle member 
of the English oolitic series. These rocks are developed to a 
great thickness in the Grraian Alps, and are identified in places 
by fossils ; but they are very much changed and the stratification 
is rarely observable. Below this occurs (2) another large series 
of rocks, among which gypsum is very frequent and remarkable. 
The gypsum is associated with clays containing here and there 
organic remains of animals, and the whole appears to represent 
the lower oolite of England, including the great and inferior 
oolites and the liassic sands. Underneath this series we have (3) 
a very remarkable and persistent group of schists, steaschists, 
talcoseschists, and other varieties of schist, with numerous 
alternations of quartz and occasional masses of quartzite. In 
the small fissures and cavities of this rock are crystals of calcite, 
sometimes very beautiful and perfect. Much of the schist is 
very hard and compact, of a dark grey or blackish colour 
streaked by numerous quartzy threads, but the variety of 
appearance is very great. On the surface many of the schists 
weather into a paler grey and very fissile rock rapidly decom- 
posing into a powdery mass, resembling at a distance a heap of 
loose sand. The kind of talus thus formed contrasts singularly 
by its smoothness with the abrupt and needle-shaped appear- 
ance of that part of the rock which has not yet been weathered, 
and that rises very grandly into mountain peaks. 
The rocks thus described may be seen on the valley side 
